Flynn was a civvie at the time. A transition team has exactly zero legal power. Nat Security Advisor is not a post that has any authority to treat with foreign nations in any way shape or form, even officially. Any discussion of sanctions, beyond listening to the Russian Ambassador and not replying would have been prohibited - and Flynn KNOWS that. He's been in the exact same circumstances before, albeit with a different government.
I more or less disagree with that. The First Amendment is the authority for anyone to express their opinion on anything - no statutory authorization is required. And yes, Flynn was a civilian. So was Trump. Yet, it would be perfectly legal if Trump said -- either during the campaign or after the election -- "if elected, I intend to negotiate the following deal with Russia...." So what's the difference if that message is conveyed publicly or privately, or directly by Trump himself or indirectly by a designated subordinate?
It is technically a violation of the Logan Act for anyone not in the government to "negotiate" with a foreign government, but nobody has been prosecuted under the Logan Act in the more than 200 years of its existence even though there have been many violations, and for very good reason. Just for starters, it is likely unconstitutional. And yes, neither Flynn, Trump, nor even a run of the mill civilian has legal authority to enter into binding agreements with a foreign government. But that's not what he did. There was no legal agreement reached on anything precisely because Flynn didn't have any authority to enter into such agreements.
You don't need statutory authority simply to talk.
I really don't have a problem -- at all -- with an incoming senior security official having preliminary discussions with foreign counterparts prior to taking office, including laying out the groundwork for a future agreement. I don't see the harm.
Then he did not tell the VP the extent of the discussion - his job, regardless of what was discussed.
That's where I think he made a significant error.